sub find {
    my ($want, $n, $elems) = @_;
    return if $n == 0;
    return if $want <= 0;
    for (0..$#$elems) {
        my $e = $elems->[$_];
        if ($n == 1) {
            return $e if $e == $want;
        }
        else {
            my @f = find($want-$e, $n-1, [@$elems less $e]);
            return ($e, @f) if @f;
        }
    }
    return;
}
# get 57 from 2 numbers in the set
@answer = find(57, 2, [1,3,7,11]);